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The cultivation and production of biomass of prickly pear is of particular interest due to the fact that it can be grown in arid areas, producing higher amounts of biomass with lower inputs (water, fertilizers etc.) compared to other crops. The present study examines whether enzyme hydrolysis is capable to increase the amount of reducing sugars available to S. cerevisiae E1A strain to be used during alcoholic fermentation. Enzymatic hydrolysis was performed using the commercial enzymes Termamyl and Celluclast applied at various concentrations of dried cladodes and of dried fruit and added juice from discarded fresh prickly pears in order to enrich the quantity of sugars into the solution. The substrate used in the bioreactor experiment consisted of 100 g/L of dried cladodes with 40 g/L of dried fruit, which after the addition of the juice resulted in the synthesis of a liquid growth medium containing 48 g/L of sugars. The microorganism finally managed to produce a significant amount of endopolysaccharides inside the yeast biomass (22.12% w/w) while biomass (DCW) production was of 4.35 g/L. Ethanol at 21.54 g/L with a yield of ethanol produced per unit of sugar consumed YEtOH/S was equal to 0.49 g EtOH / g of substrate.

Biorefinery concepts that combine several valorization pathways, including both bioproducts and bioenergy/biofuels, are of high interest in order to maximize the economic value of microalgal cultivation systems. In this study, the residual biomass obtained from the microalga Chlorella vulgaris following the chemical extraction of a valuable fraction consisting of either chlorophyll, proteins, or lipids was used as feedstock for biogas production in a laboratory-scale biochemical methane potential (BMP) assay performed in batch mode. The main aim of this study is to assess the effect of chemical extraction and removal of valuable fractions on the methane yield of residual microalgal biomass. The dry weight ratio of residual biomass to total biomass was 0.54 g/g after lipid extraction, 0.62 g/g after chlorophyll extraction, and 0.42 g/g after protein extraction. The methane yields of raw microalgae as well as residual microalgal biomass after lipid, chlorophyll, or protein extraction and removal were in the range 207–237 mL CH4/g volatile solids. Differences in the methane yields were not statistically significant. Hence, the recovery and removal of valuable compounds did not have a significant impact on the final methane yields of microalgal biomass by anaerobic digestion. Nevertheless, the methane production kinetics of residual microalgal biomass after lipid extraction displayed a diauxic pattern, possibly due to the inhibition of methanogenic bacteria by chloroform used as extraction solvent. This suggests that solvent toxicity must be considered when applying anaerobic digestion as a downstream process in biomass-based biorefineries.

As crude glycerol comprises a potential substrate for microalga fermentation and value added products' biosynthesis

The main objective of this study concerns the bioconversion of agro-industrial wastes into high added value products, such as proteinaceous animal feed, using Solid State Fermentation process (SSF). For this purpose, the Olive Mill Stone Waste (OMSW) which is known to contain low amounts of proteins and a high concentration of anti-nutritional substances was used as substrate. Subsequently, OMSW was fermented with Oat Bran (OB) or Lathyrus clymenum pericarp (LP) in proportions varying from 10 %w/w to 30 %w/w, applying SSF process initiated by Pleurotus ostreatus utilizing latter's secreted enzymes for their degradation. The respective results indicated that the addition of 30 %w/w of OB, resulted in a 39% increase of the protein content at the end of fermentation (Day 21). In addition, we observed a 5-fold increase of 1,3-1,6 β-glucan content and a simultaneous decrease of unwanted lignin of 24%. The addition of 20 %w/w of LP afforded, at the end of fermentation (Day 21), an increased protein content of 57%, a 3-fold increase in 1,3-1,6 β-glucans and a decrease in lignin concentration of 13%. These findings demonstrate the potential of the LP utilization by using SSF process, for the production of high nutritional value dietary supplements for animal feed. This endeavor constitutes the first literature report for the utilization of the agro-industrial waste LP. The developed methodology herein is considered as crucial for the circular economy since it refers to the reuse of agro-industrial wastes and the production of a high added-value product.

(1) Background: Arthrospira (commonly known as Spirulina) is an edible cyanobacterium that is produced worldwide as a food supplement owing to its high nutritional value. Arthrospira displays strong potential as an important ingredient in the development of novel functional foods. Polysaccharides from Arthrospira are biologically active compounds and hence there is interest in producing biomass rich in carbohydrates. (2) Methods: A. platensis was cultivated under different degrees of phosphorus limitation in order to trigger the accumulation of carbohydrates. The biomass was then characterized in terms of its content of α- and β-glucans, total dietary fiber and monosaccharide profile. Fourier-transform infrared spectroscopy (FTIR) was used for the rapid analysis of the main biomass components. (3) Results: Phosphorus limitation resulted in an increase in carbohydrates (from 23% up to 65% dry biomass) of which 4–12% (in relation to the dry biomass) was α-glucans and 20–34% was 1.3:1.6 β-glucans, while 1.4:1.6 β-glucans were not detected. Total dietary fibers ranged from 20–32% (of dry biomass), whereas among the carbohydrates, the predominant monosaccharide was glucose (&gt, 95%). FTIR performed well when applied as a prediction tool for the main biomass components. (4) Conclusions: Since β-glucans are of particular interest as biologically active compounds, this study demonstrates that phosphorus-limited A. platensis could be a potential ingredient for the development of novel functional foods.

Dry anaerobic digestion (D-AD) generates nitrogen-rich effluents that are normally neglected in the circular bioeconomy. The high turbidity and ammonium content hamper nitrogen recovery from these effluents via biological processes, such as microalgae culture. The goal of this study was to demonstrate microalgae growth viability in high-strength D-AD effluents in order to recover nitrogen (N) as microalgae biomass. According to the experimental factorial design conducted in batch reactors, ammonium was identified as the critical inhibitory compound for microalgae growth while turbidity did not exhibit a significantly negative effect. Instead, turbidity resulted advantageous since it promoted high nitrogen uptake rates and biomass production. The presence of organic turbidity resulted in a positive effect that boosted Chlorella growth in a stream with higher ammonium (350 mg NH

Brewers’ Spent Grain (BSG) represents the 85% of the total residue produced during the beer brewing process, with a global annual production volume exceeding the 30 Mtons. Study herein concerns the application of solid state fermentation (SSF) process for the efficient transformation of BSG into high nutritional value animal feed. The investigated SSF procedure was initiated by Pleurotus ostreatus, which constitutes a natural source of β-glucans and metabolites (vitamins, nutrients). Thus, it is possible to reduce the environmental impacts caused by BSG production and simultaneously contribute to the tackling of proteinaceous animal feed shortage observed during the last decade. The method developed resulted in a significant increase of protein content by 49.49%, a 10-fold increase of their 1,3 − 1,6 β-glucans content and a respective reduction of cellulose content by 11.42%. The application of this method is expected to provide a solid background for the utilization of BSG as substrate for fungi initiated SSF, a bioprocess allowing the significant reduction of the environmental impact caused by the beer brewing industry and simultaneously produce animal feed with high protein content and nutritional characteristics suitable for fulfilling animals’ nutritional needs and improving their welfare.

Swine wastewater (SW) poses a great threat to the environment due to its high-nutrient profiles if not properly managed. Advanced biological treatment method is an efficient method to treat SW by screening potent microalgae or bacterial strains. In this study, activated sludge, nine locally isolated heterotrophic nitrification bacteria and one microalgal strain (Chlorella) were used as inoculums in treating a local SW. Their treatment efficiencies were compared, while the nitrogen removal mechanisms and microbiome profile were explored in detail. It was found that certain heterotrophic nitrification strains had a slight advantage in removing chemical oxygen demand and phosphorus from SW, with the highest removal efficiencies of 83.9% and 76.2%, respectively. The removal efficiencies of ammonia nitrogen and total nitrogen in wastewater by microalgae reached 80.9% and 66.0% respectively, which were far higher than all the heterotrophic nitrification strains. Biological assimilation was the main pathway of nitrogen conversion by microalgae and heterotrophic nitrifying bacteria; especially microalgae showed excellent biological assimilation performance. Correlation analysis showed that Comamonas was highly positively correlated with nitrogen assimilation, while Acidovorax was closely correlated with simultaneous nitrification and denitrification. This study gives a comparison of microalgae and heterotrophic nitrifying bacteria on the nitrogen transfer and transformation pathways.

This study examined the effects of monochromatic illumination (blue, red, green and yellow) employing light-emitting diodes (LEDs), trophic conditions (photoautotrophic and mixotrophic), and nitrogen availability (high and low peptone concentration) on the growth and biochemical composition of Auxenochlorella protothecoides. The results revealed that mixotrophic conditions did not favor A. protothecoides, giving lower growth rates compared to heterotrophy (dark conditions). However, mixotrophy gave significantly higher growth rates compared to photoautotrophy. The best light wavelengths for mixotrophic cultivation were that of white and red. In all cases investigated in this study, high peptone concentration (4 g/L) resulted in decreased growth rates. Regarding the biochemical composition of A. protothecoides, the strongest effect, irrespective of trophic conditions, was caused by nitrogen availability (peptone concentration). Specifically, at nitrogen replete conditions (4 g/L peptone), biomass was rich in proteins (32–67%), whereas under deplete conditions (0.5 g/L peptone), A. protothecoides accumulated mainly carbohydrates (up to 56%). Mixotrophic conditions generally favored higher carbohydrate content, whereas photoautotrophic conditions favored higher protein content. The different illumination spectra did not have any clear effect on the biochemical composition (metabolites content), except that, in all trophic conditions, the use of the green spectrum resulted in higher chlorophyll b content. Chlorophyll a fluorescence studies revealed that the trophic conditions and the high peptone concentrations impacted the photosystem II (PSII) performance, and also affected plastoquinone re-oxidation kinetics and the heterogeneity of the PSII reaction centers.

Cultivation of photosynthetic microorganisms in wastewater is a potential cost-effective method of treating wastewater and simultaneously providing the essential nutrients for high-value biomass production. This study investigates the cultivation of the cyanobacterium Arthrospira platensis in non-diluted and non-pretreated brewery wastewater under non-sterile and alkaline growth conditions. The system’s performance in terms of biomass productivity, pollutant consumption, pigment production and biomass composition was evaluated under different media formulations (i.e., addition of sodium chloride and/or bicarbonate) and different irradiation conditions (i.e., continuous illumination and 16:8 light:dark photoperiod). It was observed that the combination of sodium bicarbonate with sodium chloride resulted in maximum pigment production recorded at the end of the experiments, and the use of the photoperiod led to increased pollutant removal (up to 90% of initial concentrations) and biomass concentration (950 mg/L). The composition of the microbial communities established during the experiments was also determined. It was observed that heterotrophic bacteria dominated by the phyla of Pseudomonadota, Bacillota, and Bacteroidota prevailed, while the cyanobacteria population showcased a dynamic behavior throughout the experiments, as it increased towards the end of cultivation (relative abundance of 10% and 30% under continuous illumination and photoperiod application, respectively). Overall, Arthrospira platensis-based cultivation proved to be an effective method of brewery wastewater treatment, although the large numbers of heterotrophic bacteria limit the usage of the produced biomass to applications such as biofuel and biofertilizer production.

Microalgae (including cyanobacteria) have recently gained interest as a potential food and feed source due to their high nutritional value. Some microalgae and cyanobacteria have been traditionally consumed worldwide. Under the European Union’s legislation, they are considered, with only few exceptions, as novel foods, and therefore in order to enter the EU market it is required primarily to be authorized after risk assessment to ensure that they do not display any risk to public health. This chapter gives an overview of the current legislative situation of microalgae as food in the European Union and discusses the potential health risks associated with the consumption of microalgae and cyanobacteria.

Anaerobic Digestion (AD) is a process that is well-known and fast-developing in Europe. AD generates large amounts of digestate, especially in livestock-intensive areas. Digestate has potential environmental issues due to nutrients (such as nitrogen) lixiviation or volatilization. Using liquid digestate as a nutrient source for microalgae growth is considered beneficial because digestate could be valorized and upgraded by the production of an added value product. In this work, microalgal biomass produced using liquid digestate from an agricultural biogas plant was investigated as a slow-release fertilizer in tomatoes. Monoraphidium sp. was first cultivated at different dilutions (1:20, 1:30, 1:50), in indoor laboratory-scale trials. The optimum dilution factor was determined to be 1:50, with a specific growth rate of 0.13 d&minus, 1 and a complete nitrogen removal capacity in 25 days of culture. Then, outdoor experiments were conducted in a 110 dm3 vertical, closed photobioreactors (PBRs) in batch and semi-continuous mode with 1:50 diluted liquid digestate. During the batch mode, the microalgae were able to remove almost all NH4+ and 65 (&plusmn, 13) % of PO43&minus, while the microalgal growth rate reached 0.25 d&minus, 1. After the batch mode, the cultures were switched to operate under semi-continuously conditions. The cell densities were maintained at 1.3 &times, 107 cells mL&minus, 1 and a biomass productivity around 38.3 mg TSS L&minus, 1 d&minus, 1 during three weeks was achieved, where after that it started to decline due to unfavorable weather conditions. Microalgae biomass was further tested as a fertilizer for tomatoes growth, enhancing by 32% plant growth in terms of dry biomass compared with the control trials (without fertilization). Similar performances were achieved in tomato growth using synthetic fertilizer or digestate. Finally, the leaching effect in soils columns without plant was tested and after 25 days, only 7% of N was leached when microalgae were used, against 50% in the case of synthetic fertilizer.

Aquatic organisms, such as microalgae (Chlorella, Arthrospira (Spirulina), Tetrasselmis, Dunalliela etc.) and duckweed (Lemna spp., Wolffia spp. etc.) are a potential source for the production of protein-rich biomass and for numerous other high-value compounds (fatty acids, pigments, vitamins etc.). Their cultivation using agro-industrial wastes and wastewater (WaW) is of particular interest in the context of a circular economy, not only for recycling valuable nutrients but also for reducing the requirements for fresh water for the production of biomass. Recovery and recycling of nutrients is an unavoidable long-term approach for securing future food and feed production. Agro-industrial WaW are rich in nutrients and have been widely considered as a potential nutrient source for the cultivation of microalgae/duckweed. However, they commonly contain various hazardous contaminants, which could potentially taint the produced biomass, raising various concerns about the safety of their consumption. Herein, an overview of the most important contaminants, including heavy metals and metalloids, pathogens (bacteria, viruses, parasites etc.), and xenobiotics (hormones, antibiotics, parasiticides etc.) is given. It is concluded that pretreatment and processing of WaW is a requisite step for the removal of several contaminants. Among the various technologies, anaerobic digestion (AD) is widely used in practice and offers a technologically mature approach for WaW treatment. During AD, various organic and biological contaminants are significantly removed. Further removal of contaminants could be achieved by post-treatment and processing of digestates (solid/liquid separation, dilution etc.) to further decrease the concentration of contaminants. Moreover, during cultivation an additional removal may occur through various mechanisms, such as precipitation, degradation, and biotransformation. Since many jurisdictions regulate the presence of various contaminants in feed or food setting strict safety monitoring processes, it would be of particular interest to initiate a multi-disciplinary discussion whether agro-industrial WaW ought to be used to cultivate microalgae/duckweed for feed or food production and identify most feasible options for doing this safely. Based on the current body of knowledge it is estimated that AD and post-treatment of WaW can lower significantly the risks associated with heavy metals and pathogens, but it is yet unclear to what extent this is the case for certain persistent xenobiotics.

Wastewater treatment technology with better energy efficiency and recyclability is in urgent demand. Photo-Sequencing batch reactor (SBR), which introduces microalgae into conventional SBR, is considered to have more potential for resource recycling. In this study, a photo-SBR was evaluated through the manipulation of several key operational parameters, i.e., aeration strength, light supply intensity and time per cycle, and solid retention time (SRT). The algal-bacterial symbiotic system had the potential of removing COD, NH4+-N and TN with limited aeration, representing the advantage of energy-saving by low aeration requirement. Maintaining appropriate proportion of microalgae in the symbiotic system is critical for good system performance. Introducing microalgae into conventional SBR has obvious impact on the original microbial ecology. When the concentration of microalgae is too high (>4.60 mg Chl/L), the inhibition on certain phyla of bacteria, e.g., Bacteroidetes and Actinobacteria, would become prominent and not conducive to the stable operation.

The interactive effects of light intensity, NaCl, nitrogen, and phosphorus on intracellular biomass content and extracellular polymeric substance production were assessed for Arthrospira sp. (Spirulina) in a two-phase culture process using principal component analysis and central composite face design. Under high light intensity (120 μmol photons m−2 s−1) and low NaCl (1 gL−1), NaNO3, and K2HPO4 (0.5 g L−1), the carbohydrate content was maximized to 26.61%. Interaction of both K2HPO4 (1.6 gL−1) and NaCl (1.19 gL−1) with low NaNO3 (0.5 gL−1) achieved the maximum content of lipids (15.62%), while high NaCl (40 gL−1), K2HPO4, and NaNO3 (4.5 gL−1) enhanced mainly total carotenoids (0.85%). Conversely, under low light intensity of 10 μmol photons m−2 s−1 combined with 11.76 gL−1 of NaCl, 0.5 gL−1 of NaNO3, and 2.68 gL−1 of K2HPO4, the phycobiliprotein content reached its highest level (16.09%). The maximum extracellular polymeric substance (EPS) production (0.902 gg−1 DW) was triggered under moderate light of 57.25 μmol photons m−2 s−1 and interaction of high NaCl (40 gL−1) and K2HPO4 (4.5 gL−1) with low NaNO3 (0.5 gL−1). The maximization ratios of intracellular biomass content in terms of carbohydrate, lipid, total carotenoid, phycobiliprotein, and EPS production were 3.55-, 1.73-, 9.55-, 2.92-, and 1.46-fold, respectively, greater than those obtained at optimal growth conditions. This study demonstrated that the multiple stress factors applied to the adopted two-phase culture process could be a promising strategy to produce biomass enriched in various high-value compound.

Nitrogen (N) limitation is considered as the most efficient strategy to induce the accumulation of lipids, carbohydrates or other target compounds in microalgal biomass. However, along with biomass biochemical composition, alterations in N limitation affect the photosynthetic apparatus and result in decreased growth. In this study, Chlorella vulgaris was cultivated in semi-continuous mode with different degrees of N limitation and chlorophyll (Chl) fluorescence analyses were used to investigate the effect of N limitation on photosystem II (PSII) performance, in terms of structural and functional heterogeneity. As expected, N limitation resulted in the decrease of quantum yield and calculated OJIP parameters related to PSII performance. N limitation resulted in a significant increase of trapped energy per reaction center (RC) and subsequently to higher dissipation of excess energy. However, despite the negative effect of N limitation on the number of RCs, the electron transport beyond QA as well the capacity of reducing/re-oxidation of plastiquinone were not negatively affected, implying that performance of RCs was not affected by N limitation. Photochemical quenching (qp) increased of as N limitation increased while the curve of non-photochemical quenching (NPQ) was unimodal, i.e. increased up to a level of N limitation and then decreased as N limitation degree increased further. The overall results of the present study suggest that the decrease of PSII performance was due to a reduction of the number of RCs accompanied with higher energy dissipation a probable outcome of the decreased need for reductant by cells due to lower metabolic activity under N limitation.

The detection of new pollutants, stricter environmental regulations, and advancements in treatment technologies are driving improvements in bioprocesses for treating wastewater. Specifically, special concern is being placed on phosphorus and nitrogen forms, which spur eutrophication of water bodies, and emerging micropollutants such as pharmaceuticals and person-care products. Algae and cyanobacteria cultivation requirements include water and nutrient sources that currently are supplied by fertilizers, which provide poor sustainability and economics. Using wastewater as a source of nitrogen and phosphorus represents an attractive option to cultivate microalgae simultaneous with contaminant removal. Phycoremediation refers to the assimilation or disintegration of organic and inorganic compounds (carbon, nitrogen, or phosphorus), metals, and emerging contaminants in wastewater by microalgae – algae and cyanobacteria. In addition, added value comes when the microalgae are harvested to become feedstock for biofuels such as biogas. Although promising studies have been published for algal growth in wastewater while simultaneously removing contaminants, limitations in the scale-up process still have to be addressed. In this work, we summarize biological mechanisms by which nutrients and contaminants are removed by microalgae. We specifically address the interactions that the microalgae have with other microorganisms and the production of extracellular polymeric substances, a mechanism well known in the literature, but hardly studied in microalgae. Finally, we discuss different strategies reported to improve the scale up of microalgae cultivation in wastewater.

Microalgae are photosynthetic aquatic microorganisms that fix CO2 and other inorganic elements into organic matter. In general terms, microalgae are considered as a very interesting source of biomass because of their great diversity on biochemical and physiological properties and their convenient agronomical characteristics, like high biomass yield, fast grow, year-round harvesting capability, independence from arable land or irrigation-quality water. However, there are some important bottlenecks on the large-scale cultivation of microalgae and specific challenges exist. Currently there is a vast research activity trying to address these bottlenecks. The most challenging and critical points are related to the low biomass concentration on the cultivation medium, high energy consumption and costs for fertilization, agitation, harvesting and drying of biomass. This chapter aims to give an introductory overview on microalgal biomass production highlighting the potentials and limitations of microalgal cultivation, biomass production, biomass processing and application.

Herein a techno-economic assessment was performed on an energy-crop-based biogas plant coupled with a greenhouse for utilizing thermal energy produced by cogeneration. Seven energy crops were evaluated: triticale, maize, alfalfa, sunflower, clover, barley and wheat. According to the evaluation, triticale was the most competitive energy crop under selected climate conditions for northern Greece. Although maize displays higher biomass yield and biogas potential than the drought-resistant crop triticale, it has high irrigation demand that contributes significantly to total production costs. For a triticale-based biogas production to become economically feasible, agricultural arable area larger than 500 ha, or biogas plant size larger than 1000 kWel, is required. However, with public funding, biogas production becomes feasible at smaller area (>250 ha) or biogas plant size (>500 kWel). The inclusion of a greenhouse into the design of the biogas plant contributes positively to the economic viability of the entire system. Under this scenario, greenhouse financial income accounts for about 17–18% of total income. Results of a sensitivity analysis suggest that the selection of an appropriate energy crop for biogas production should be based principally on both digestibility (specific methane yield) and biomass yield per hectare, these factors being more critical than biomass production costs.

The biosorption of phosphorus in the form of orthophosphate (Po) from wastewater using biomass as the sorbent is of potential importance because the Po-loaded biomass could be applied in the agricultural sector as fertilizer and soil conditioner. However, biomass generally displays a very low affinity for Po sorption and therefore biomass surface modification is required. In the present study, the biomass (as model grinded leaves of Phragmites sp. were used) was pretreated with Ca(OH)2 to enhance Po biosorption capacity (qe). The results indicate that the alkaline pretreatment resulted in a modification of surface functional groups. It was concluded that the main sorption mechanisms were ligand exchange and electrostatic attraction. A series of experiments were conducted to investigate the performance of the pretreated biomass for Po uptake under various conditions. Isotherm and thermodynamic studies were also applied and analyzed. The biosorption process was best described by the pseudo-second order kinetic model and Langmuir isotherm, which gave a qmax of 12.27mgP/g at 25°C and pH7. The Ca(OH)2 treated Phragmites biomass applied in this study for Po recovery may present some potential advantages in terms of costs and environmental impact.

Although ammoniacal nitrogen is the preferred nitrogen source for microalgae/cyanobacteria, at elevated concentrations and high pH values it may negatively impact photosynthesis and growth. Chlorophyll (Chl) fluorescence analysis is a useful tool to monitor the influence of various stress conditions on the photosynthetic activity of plants or microalgae/cyanobacteria. In this study, we investigated the effect of ammoniacal nitrogen on Chl fluorescence in microalgae/cyanobacteria. Chl fluorescence analysis revealed that the parameters related to flux ratios and specific energy fluxes of photochemistry were gradually inhibited as the free ammonia (FA) concentration increased. Photosynthetic electron transport activity was measured using artificial electron acceptors, donors or inhibitors. These analyses suggest that ammonia has multiple impacts on the photosynthetic apparatus; photosystems I (PSI) and II (PSII), the electron transport chain, the oxygen-evolution complex (OEC) as well the dark respiration were gradually inhibited by increasing FA concentration. At high FA concentration, the PSI/PSII activity increased, suggesting that PSI was more tolerant to FA than PSII. Non-photochemical quenching (NPQ) decreased to zero at elevated FA concentrations. The Chl fluorescence data obtained in the presence of DCMU (diuron) suggest that the decrease of NPQ under ammonia inhibition/toxicity is due to the increase of PSI/PSII activity. The rapid response of Chl fluorescence transients to increases in FA may allow one to use pulse amplitude modulation (PAM) fluorescence as a tool to monitor ammonia inhibition/toxicity in cultures of microalgae/cyanobacteria.

Utilization of digestate as a source of nutrients must be understood in the context of their effects on light penetration. These conflicting consequences of digestate utilization were verified by growing the cyanobacterium Arthrospira platensis (Spirulina) in artificial seawater supplemented with 2.5 %, 5%, 10 % or 15 % (v/v) of anaerobically digested effluents (digestate) in a fed-batch mode. Therefore, the study investigates the impact of the complex interactions between nutrient availability, increased salinity, which affects negatively the photosynthetic efficiency of A. platensis and variable turbidities associated with the variable ranges of digestate additions. At the lowest digestate concentration growth was hindered by low nutrient availability. Biomass production was highest at 5% digestate (≈1300 mg/L dry biomass) but further increases in digestate concentrations lowered biomass accumulation (≈1020−1150 mg/L dry biomass). On the other hand, greater concentrations of digestate and thus more nutrients, led to greater protein content (44–55 % in higher concentrations versus 22−30% in the lowest), while A. platensis cells had enhanced photosynthetic performances. The latter is likely due to the fact that addition of digestate reduced light intensity and therefore reduced the negative effect of salinity on the photosynthetic apparatus. The balance between nutrient and light availability, as associated with variable digestate rates, may be managed towards the production of A. platensis biomass with variable qualitative and quantitative biochemical parameters.

Anaerobic digestion (AD) is a widely used technology for the treatment of organic wastes and by-products. Through AD, the organic matter is degraded producing a gaseous stream (biogas, which is a mixture of mainly methane and carbon dioxide) and a liquid/slurry stream (digestates) that contains most of the mineralized elements originating in the feedstock. Digestates are a very interesting source of nutrients for growing microalgae to produce valuable biomass with a simultaneous further treatment of the digestates. The performance of microalgae grown using digestates is influenced by various cultivation parameters, such as the physicochemical characteristics of the digestates (nutrient profile, content of inhibitory compounds, etc.), light penetration (turbidity and colored dissolved compounds), mixing regime, and hydraulic retention time (HRT). Digestates are characterized by their high content in ammoniacal nitrogen, suspended solids, and several inhibitors that might limit growth, and therefore pretreatment of digestates is likely to have a positive effect on biomass production. Microalgal cultivation is proven as an efficient technology for the removal of nitrogen, phosphorus, organic load, and other contaminants (heavy metals, pathogens). The produced biomass could be used as feedstock for the production of various commodities (biofuels, feed, etc.); however there are some concerns about the potential contamination of microalgal biomass with unwanted hazardous pollutants. This book chapter aims to give an overview on the cultivation of microalgae utilizing digestates derived from agro-industrial wastes and by-products, discussing the potentials and the drawbacks of such an approach.

Microalgae are an interesting new generation feedstock for the production of biofuels. However, their production costs are currently much higher than fossil fuels, restricting their commercial production. Among the various production processes, the consumption of fertilizers (carbon, nitrogen, phosphorus, etc.) contributes significantly to the total production costs. Moreover, it is projected that the use of industrially manufactured fertilizers will outcome in an unsustainable process, also raising competition between biofuels and food-crops for nutrient availability. To improve the economics and the sustainability of microalgal biofuels, an approach for recycling of nutrients from waste streams should be established, in order to apply nutrients from waste/wastewater at low cost, and to avoid using industrially manufactured fertilizers. For this purpose, a closed-loop production scheme should be developed, in which a series of technologies will be employed in order to obtain the main products (biofuels and other biorefinery products) along with a stream rich in spent nutrients for their reuse for further microalgae cultivation and biomass production.

In this chapter, the biological treatment process of wastewater and biomass, called anaerobic digestion, is discussed. It is a potential bioprocess to produce renewable energy as methane and hydrogen from underestimated and unexploited sources of organic matter. More specifically, this chapter will discuss the types of biodigesters utilized, the operation modes and the main parameters that affect the process, aiming to provide the knowledge to achieve process stability and reproducibility. The operation strategies, such as substrates co-digestion and two-stage process, and biomass pretreatment will be as well discussed in detail. This chapter will focus on some Brazilian agro-industrial effluents (vinasse and manipueira), as well as food wastes, lignocellulosic biomass, and micro/macroalgae for biogas production. To conclude, the overall objective of the chapter is to give general information and possibilities to apply and conduct the anaerobic digestion process.